Electrolytic solution

ABSTRACT

An electrolytic solution for electrolytic capacitors is disclosed which allows to generate higher spark voltages without inducing decrease in the specific electric conductivity. The electrolytic solution comprises an organic polar solvent (C) and further comprises a carboxylic acid (A0) and/or its salt (B) dissolved in the solvent, wherein the carboxylate anion (A) is characterized in that the energy of formation in water of its ionic complex (D) with aluminum ion is not more than −250 kcal/mol and not less than −500 kcal/mol as calculated by the MM3/PM3 method of the CAChe system.

TECHNICAL FIELD

The present invention relates to an electrolytic solution, morespecifically to an electrolytic solution to be used in electrolyticcapacitors.

BACKGROUND ART

Along with progression of space-saving around installed capacitors inrecent years, there has been arising need for an electrolytic solutioncapable of generating high spark voltages that will enable down-sizingof capacitors. For this, an electrolytic solution is proposed in whichis utilized a polycarboxylic acid having two or more secondary and/ortertiary carboxyl groups in total and whose molecular weight is not lessthan 260 (see, e.g., Japanese Patent Application Publication No.H1-103821).

In order to increase spark voltages, either of the following two meanshas been employed so far, i.e., increasing the molecular weight, thusincreasing the van der Waals volume, of the anionic components in theelectrolytic solution, or employing additives such as ethylene glycol.However, by either of these means, decrease in specific electricconductivity occurs inversely proportional to the increase in sparkvoltages.

DISCLOSURE OF INVENTION

The purpose of the present invention is to obtain an electrolyticsolution that enables generation of high spark voltages without inducinga decrease in the specific electric conductivity.

In general, spark voltage strongly correlates to the van der Waalsvolume of the anionic ingredients of an electrolytic solution: thegreater the van der Waals volume, the higher the spark voltage. On theother hand, specific electric conductivity is apt to decrease when thevan der Waals volume of an anionic ingredient increases. However,depending on their molecular structures, there are some cases wherediscrepancy in spark voltages exists between molecules comparable toeach other in terms of van der Waals volume and specific electricconductivity. The present inventors herein assumed the presence of asecond factor, apart from the van der Waals volume of an anionicingredient, the factor contributing to spark voltages without adverselyaffecting specific electric conductivity, and thus set to a study. As aresult, the factor was pinned down to be the energy of formation inwater of the ionic complex from the carboxylate anion and aluminum ion,which energy may be calculated by the MM3/MP3 method of the CAChesystem, and this lead to the present invention. Thus, the presentinvention is an electrolytic solution comprising an organic polarsolvent (C) and further comprising a carboxylic acid (A0) and/or acarboxylate salt (B), wherein the carboxylate anion (A) is one withwhich the energy of formation in water of the ionic complex (D) withaluminum ion is not more than −250 kcal/mol and not less than −500kcal/mol as calculated by the MM3/PM3 method of the CAChe system.

Using the electrolytic solution for electrolytic capacitors according tothe present invention, higher spark voltages can be generated withoutinducing a decrease in the specific electric conductivity.

BEST MODE FOR CARRYING OUT THE INVENTION

The carboxylate anion (A) derived from the carboxylic acid (A0) and/orthe carboxylate salt (B), components of the electrolytic solution of thepresent invention, is one with which the energy of formation in water ofthe coordination complex (D) with aluminum ion is usually not more than−250 kcal/mol, preferably not more than −300 kcal/mol, more preferablynot more than −350 kcal/mol, and usually not less than −500 kcal/mol,preferably not less than −450 kcal/mol, as calculated by the MM3/PM3method of the CAChe system.

When the energy of formation is above −250 kcal/mol or below −500kcal/mol, the effect of increasing spark voltages turns small. It is thesmallness of the energy of formation of the ionic complex (D) of thecarboxylate anion (A) with aluminum ion that enables higher sparkvoltages without requiring increase in the van der Waals volume of thecarboxylate anion, thus giving an electrolytic solution simultaneouslyhaving high specific electric conductivity and high spark voltage.Though the reason is not clear, the increase in spark voltage isconsidered to be due to suppressed diffusion of aluminum ion into theelectrolytic solution because of the formation of the ionic complex (D),and to efficient restoration of anodic oxidation-formed coating filmsfrom partial damage.

The energy of formation may be calculated using software produced byFujitsu Limited, “CAChe4.4”, by inputting the structure of D andselecting the MM3/PM3 method as the method for calculation.

As for the carboxylate anion (A), dicarboxylate anion is preferred, forit more readily coordinates with aluminum ion and thus leads toreduction of the energy of formation.

The carboxylate anion (A) employed in the present invention is one thatis characterized in that the energy of formation in water of the ioniccomplex (D) it forms with aluminum ion is not more than −250 kcal/mol ascalculated by the MM3/PM3 method of the CAChe system.

In the present invention, “aluminum ion” means the trivalent cationAl³⁺. While the van der Waals volume of the carboxylate anion (A)employed may be in a proper range corresponding to required sparkvoltages and specific electric conductivities, from a viewpoint ofobtaining an electrolytic solution suitable for capacitors ofmoderate/high voltage classes over 100 V, the volume is preferably notless than 190 cubic angstroms, more preferably not less than 250 cubicangstroms, and preferably not more than 500 cubic angstroms, morepreferably not more than 400 cubic angstroms. Herein, the van der Waalsvolume, which may be calculated using the CAChe system, means the volumeof a solid defined by connecting the points at which the sameprobability density of electron is exhibited in the three-dimensionalspace and having an energy of 18 kcal/mol.

The modulus of the difference between the solubility parameter(hereinafter abbreviated to “SP value”) for the carboxylate anion (A) ascalculated by Fedors method and the SP value for the organic polarsolvent (C) is preferably not less than 4, and preferably not more than9, more preferably not more than 8, from a viewpoint of the solubilityof the electrolyte in the solvent.

The SP value for the carboxylate anion (A) as calculated by Fedorsmethod is the value calculated by the method described in PolymerEngineering and Science, Vol. 14, No. 2, p. 147˜154(1974). Namely, SPvalue δ (at 25° C.) is given by the following formula:$\delta = {\left( \frac{\Delta\quad E}{V} \right)^{1/2} = \left( \frac{\sum\limits_{i}{\Delta\quad e_{i}}}{\sum\limits_{i}{\Delta\quad v_{i}}} \right)^{1/2}}$

-   -   where ΔE and V: cohesive energy density and molar volume,        respectively,    -   Δe_(i) and Δv_(i): energy of vaporization and molar volume,        respectively, for an atom or a group of atoms,    -   provided that, for resins having Tg>25° C., the following values        are added to the molar volume:    -   if n<3, then +Δv_(i)=4n    -   if n≧3, then +Δv_(i)=2n    -   n: the number of the atoms forming the principal chain of the        smallest repeating unit building up the polymer.

As for the carboxylate anion (A) employed in the present invention, themaximal electron density of the highest occupied orbital of the molecule(A), as calculated by the MM3/PM3 method of the CAChe system, ispreferably not less than zero, preferably not more than 0.5, and morepreferably not more than 0.3, from a viewpoint of stability in terms ofthe breakage of the molecule upon application of voltage and exposure tothermal history.

Within this range, the carboxylate anion (A) resists molecular breakagethat could be induced by electrical and thermal energy.

As for the carboxylate anion (A), secondary dicarboxylate dianion ismore preferred from a viewpoint that it can suppress esterificationreaction with the solvent and its decline in the specific electricconductivity is small at high temperatures (100° C.-180° C.). This isdue to increased steric hindrance in secondary dicarboxylic acid becauseof a substituent at the α-position relative to the carboxyl group.

The carboxylate anion (A) is one that is represented by the followinggeneral formula (1)(A1) or one that is represented by the followinggeneral formula (2)(A2). Among (A1) and (A2), those that are preferredare a dicarboxylate dianion having an ether bond, a dicarboxylatedianion having a hydroxyl group, and a dicarboxylate dianion havingasymmetrical side chains.

-   -   wherein X is a linear or branched, saturated or unsaturated,        divalent hydrocarbon group having 1 to 12 carbon atoms, and,        from a viewpoint of simultaneous achievement of high specific        electric conductivity and high spark voltage, the number of        carbon atoms is preferably not less than 2, more preferably not        less than 4, and preferably not more than 10, and more        preferably not more than 8. X may have an ether bond. Examples        of X include such groups as ethylene, propylene, isopropylene,        butylene, isobutylene, pentylene, hexylene, heptylene, octylene,        a group represented by —O[CH₂]_(n)—O— (n is 1-10), a group        represented by —OC₆H₄—O—, and polyoxyalkylene (polymerization        degree of 2-4; ethylene or isopropylene for the alkylene). Among        them, particularly preferred are hexylene, heptylene, octylene        and polyoxyalkylene groups.

R₁ and R₂ are linear or branched, saturated or unsaturated, monovalenthydrocarbon groups having 2 to 10 carbon atoms wherein R₁ and R₂ aredifferent from each other, or R₁ and R₂ are monovalent hydrocarbongroups having an ether bond and having 2 to 10 carbon atoms wherein R₁and R₂ may be the same or different from each other. Examples of R₁ andR₂ include such groups as ethyl, propyl, isopropyl, butyl, isobutyl,pentyl, hexyl, heptyl, octyl, 2-ethylhexyl, methylpolyoxyalkylene(polymerization degree of 2-4; ethylene or isopropylene for thealkylene), and phenylpolyoxyalkylene (polymerization degree of 2-4;ethylene or isopropylene for the alkylene). Among those, particularlypreferred are isopropyl, hexyl, heptyl, octyl, and 2-ethylhexyl groupswhere R₁ and R₂ are different with each other, or methylpolyoxyalkyleneand phenylpolyoxyalkylene, from a viewpoint of simultaneous improvementof solubility to solvents and withstand voltage.

Y is a linear or branched, saturated or unsaturated, divalenthydrocarbon group having 1 to 12 carbon atoms, and, from a viewpoint ofsimultaneous achievement of high specific electric conductivity and highspark voltage, the number of carbon atoms is preferably not less than 2,more preferably not less than 4, and preferably not more than 10, morepreferably not more than 8, and an ether bond may be included. Examplesof Y include such groups as ethylene, propylene, isopropylene, butylene,isobutylene, pentylene, hexylene, heptylene, octylene, a grouprepresented by —O[CH₂CH₂]_(n)—O— (n is 1-5), a group represented by—O[C₆H₆]—O—, and polyoxyalkylene (polymerization degree of 2-4; ethyleneor isopropylene for the alkylene). Among those, particularly preferredare hexylene, heptylene, octylene, and polyoxyalkylene groups.

R₃ and R₄ are linear or branched, saturated or unsaturated, divalenthydrocarbon groups having 1 to 10 carbon atoms, and may have an etherbond. R₃ and R₄ may be the same or different from each other. Examplesof R₃ and R₄ include such groups as ethylene, propylene, isopropylene,butylene, isobutylene, pentylene, hexylene, heptylene, octylene, a grouprepresented by —[OCH₂CH₂]_(n)— (n is 1-4), and a group represented by—[OCH₂CH(CH₃)]_(n)— (n is 1-3). Particularly preferred are hexylene,heptylene, and octylene groups.

Examples of methods for preparing the aforementioned carboxylate anion(A1) include; a method in which, applying the technique described in theJournal of Organic Chemistry, 24,54(1959), a carboxylate ester having anethylene oxide ring, which is prepared from an acrylate ester andhydrogen peroxide, is reacted with a linear and/or branched, saturatedand/or unsaturated diol having 1 to 10 carbon atoms and which may havean ether bond, and further reacted with ethylene oxide and/or propyleneoxide, and then subjected to saponification in a conventional method togive the aimed compound; and a method in which polyethylene glycol isreacted with p-toluenesulfonyl chloride, subsequently subjected tonucleophilic addition with methylpolyethylenemalonic acid methyl orethyl ester having 6-17 carbon atoms at a reaction temperature of 70° C.in the absence of a solvent, and then to saponification anddecarboxylation by a conventional method.

An example of methods for preparing the aforementioned carboxylate anion(A2) is a method in which 1,6-dicyclohexane is subjected to nucleophilicaddition reaction with polyoxyethylenemalonic acid methyl or ethyl esterof a polymerization degree of 1-4 at a reaction temperature of 70° C. inthe absence of a solvent, then to saponification and decarboxylation bya conventional method.

The aforementioned carboxylate anion (A2) may also be obtained byanother method, e.g., in which polyethylene glycol is reacted withp-toluenesulfonyl chloride, then subjected to nucleophilic displacementreaction with alkylmalonic acid methyl or ethyl ester having 6-17 carbonatoms at a reaction temperature of 70° C. in the absence of a solvent,and to saponification and decarboxylation by a conventional method.

The carboxylic acid (A0) is the protonated form of the correspondingcarboxylic anion (A). From a viewpoint of reactivity with solventmolecules and solubility in the solvent, preferred carboxylic acid (A0)is a secondary dicarboxylic acid, and especially preferred is asecondary dicarboxylic having a hydrophilic group such as a hydroxylgroup or an ether bond in the molecule. More specifically, it is theprotonated form of what is represented above by the general formula(1)(A1) or the general formula (2)(A2).

Examples of the carboxylate salt (B) include ammonium salts and aminesalts of the carboxylic acid (A0).

Examples of amines (bases) which form the amine salts include primaryamines (methylamine, ethylamine, ethylenediamine, etc.), secondaryamines (dimethylamine, diethylamine, etc.), and tertiary amines[trimethylamine, triethylamine, dimethylethylamine,dimethylisopropylamine, 1,8-diazabicyclo(5,4,0)-undecene-7, etc.]. Amongthose, preferred are ammonium salts and triethylamine salts, andparticularly preferred are ammonium salts.

The molar ratio of the carboxylate group (A) to ammonium group whichform carboxylate salts (B) is preferably (1:2)-(1:0.5), and morepreferably (1:1.2)-(1:0.8).

Examples of organic polar solvents (C) used in the electrolytic solutionfor electrolytic capacitors of the present invention include one or moresolvents selected from alcohols, ethers, amides, lactones, nitriles,carbonates, and other organic polar solvents.

Specific examples of organic polar solvents (C) include the following:

(1) Alcohols;

Monovalent alcohols; monovalent alcohols having 1-6 carbon atoms (methylalcohol, ethyl alcohol, propyl alcohol, butyl alcohol, diacetonealcohol, furfuryl alcohol, etc.), and monovalent alcohols having notless than 7 carbon atoms (benzyl alcohol, octanol, etc.),

Divalent alcohols; divalent alcohols having 1-6 carbon atoms (ethyleneglycol, propylene glycol, diethylene glycol, hexylene glycol, etc.), anddivalent alcohols having not less than 7 carbon atoms (octylene glycol,etc.),

Trivalent alcohols; trivalent alcohols having 1-6 carbon atoms(glycerol, etc.),

Quadrivalent to sexivalent alcohols or alcohols having more valences;quadrivalent to sexivalent alcohols, or alcohols having more valences,having 1-6 carbon atoms (hexitol, etc.),

(2) Ethers;

-   -   Monoethers (ethylene glycol monomethyl ether, ethylene glycol        monoethyl ether, diethylene glycol monomethyl ether, diethylene        glycol monoethyl ether, ethylene glycol monophenyl ether,        tetrahydrofuran, 3-methyltetrahydrofuran, etc.), diethers        (ethylene glycol dimethyl ether, ethylene glycol diethyl ether,        diethylene glycol dimethyl ether, diethylene glycol diethyl        ether, etc.), etc.,        (3) Amides;

Formamides (N-methylformamide, N,N-dimethylformamide, N-ethyl-formamide,N,N-diethylformamide, etc.), acetamides (N-methylacetamide,N,N-dimethylacetamide, N-ethylacetamide, N,N-diethylacetamide, etc.),propionamides (N,N-dimethylpropionamide, hexamethylphosphorylamide,etc.), oxazolidinones (N-methyl-2-oxazolidinone,3.5-dimethyl-2-oxazolidinone, etc.).

(4) Lactones;

α-acetyl-γ-butyrolactone, β-butyrolactone, γ-valerolactone,δ-valero-lactone, etc.

(5) Nitriles;

Acetonitrile, acrylonitrile, etc.

(6) Carbonates;

Ethylene carbonate, propylene carbonate, etc.

(7) Other Organic Polar Solvents;

Dimethylsulfoxide, sulfolane, 1,3-dimethyl-2-imidazolidinone,N-methylpyrrolidone, etc.

Among those enumerated above, more preferred are divalent alcoholshaving 1-6 carbon atoms (ethylene glycol, propylene glycol, diethyleneglycol, hexylene glycol, etc.), and still more preferred is ethyleneglycol.

The weight content of the organic polar solvent (C) is preferably 5-95%by weight relative to the total weight of the electrolytic solution,more preferably 30-95% by weight, and most preferably 60-95% by weight.

When required, a non-polar solvent such as an aromatic solvent (toluene,xylene, etc.), and a paraffinic solvent (normal paraffin, isoparaffin)may be concomitantly used, along with an organic polar solvents (C)enumerated above, as a solvent used for the electrolytic solution forelectrolytic capacitors.

The content of such a non-polar solvent is preferably not more than 20%by weight of the total weight of the electrolytic solution.

When required, water may be included in the electrolytic solution forelectrolytic capacitors. The content is not more than 10% by weight ofthe total weight of the electrolytic solution.

As the ionic complex (D) formed from the carboxylate anion (A) andaluminum ion, particularly preferred is one represented by the formula(3) or (4).

The ionic complexes (D) shown above are hypothetical compounds used forthe calculation of the energy of formation in water. The carboxylic acid(A0) and/or carboxylate salt (B) dissolved in the organic polar solvent(C) are considered to from, through a reaction with aluminum in theelectrolytic capacitor, an ionic complex (D) with aluminum ion.

The total weight of the carboxylic acid (A0) and/or carboxylate salt (B)is preferably 1-70% by weight relative to the total weight of theelectrolytic solution, and more preferably 5-40% by weight.

When required, a variety of additives that are commonly used inelectrolytic solutions may be added to the electrolytic solution of thepresent invention.

Examples of such additives include phosphoric acid derivatives (e.g.,phosphoric acid, phosphate esters, etc.), boric acid derivatives (e.g.,boric acid, complexes of boric acid and a polysaccharide (such asmannitol, sorbitol, etc.), complexes of boric acid and a polyol(ethylene glycol, glycerol, etc.)), nitro compounds (e.g.,o-nitrobenzoic acid, p-nitrobenzoic acid, m-nitrobenzoic acid,o-nitrophenol, p-nitrophenol, etc.), etc.

When required, a small amount of carboxylic acid having a primarycarboxyl group or a carboxylic acid having an aromatic carboxyl groupmay be admixed to improve performance for coating film formation byanodic oxidation or for additional improvement of the specific electricconductivity. Examples of those which can be admixed include adipicacid, azelaic acid, 1,6-decanedicarboxylic acid, 2-butylhexanedioicacid, benzoic acid, etc.

The total amount of additives mentioned above is preferably not morethan 10% by weight of the total weight of the carboxylic acid (A0) andthe carboxylate salt (B).

The pH of the electrolytic solution of the present invention ispreferably 3-12, more preferably 5-10.

In preparing the polycarboxylic acid salt (B), conditions are selectedso that the pH of the electrolytic solution will fall within the aboveranges. Herein, the pH of the electrolytic solution is the valuemeasured of the electrolytic stock solution at 25° C.

The electrolytic solution of the present invention is used forelectrolytic capacitors, preferably for electrolytic capacitors ofmoderate/high voltage classes having spark voltages of 100 V or more.

EXAMPLES

The present invention will be described in further detail below withreference to examples. The present invention, however, is not limited tothose examples.

Preparation Example 1 Preparation of Carboxylate Ester Having an EpoxyRing

344 g (4 mol) of methyl acrylate, 478 g (4.2 mol) of 30% hydrogenperoxide and 23.2 g of sodium tungstate were charged in a 2-literfour-necked flask equipped with a rectification column, and reaction wasallowed for 2 hours at 70° C. with stirring. Subsequent rectificationgave 285.6 g of (E-1) represented by the following formula.

Example 1

118 g (1 mol) of 1,6-hexanediol and 12.1 g of boron trifluoride werecharged in a 1-liter autoclave. After heating to 65° C., 210.1 g (2.06mol) of the aforementioned (E-1) was added dropwise over 8 hours, andthen 90.6 g (2.06 mol) of ethylene oxide was added dropwise over 8hours. Extraction of the reaction mixture with ethyl ether, followed bytreatment with 10 N potassium hydroxide and then with 6 N hydrochloricacid, gave 170.4 g of the dicarboxylic acid (A0-1) represented by thefollowing formula. 18.36 g (0.05 mol) of (A0-1) thus obtained wasdissolved in 80 g of ethylene glycol, and purged with ammonia gas.Termination of purging when neutrality was reached gave a 100-g solutionof the ammonium dicarboxylate (B-1).

Example 2

The same procedure was followed as in Example 1 except that 76 g of1,3-propanediol was used in place of 1,6-hexanediol in Example 1, thatthe amount of ethylene oxide added dropwise was 135.9 (3.09 mol), andthat the duration of ethylene oxide dropwise addition was 12 hours.160.3 g of dicarboxylic acid (A0-2) represented by the formula shownbelow was thus obtained. 18.18 g (0.05 mol) of thus obtained (A0-2) wasdissolved in 80 g of ethylene glycol, purged with ammonia gas.Termination of purging when neutrality was reached gave a 100-g solutionof the ammonium dicarboxylate (B-2).

Preparation Example 2 Preparation of Ditosylated Compound fromTetraethylene Glycol

193.2 g (1 mol) of tetraethylene glycol and 900 ml of dry pyridine werecharged in a 3-liter four-necked flask. A solution of p-toluenesulfonylchloride dissolved in 600 ml of dry toluene was added dropwise withstirring while keeping the temperature of the content of the flask below10° C. Reaction was allowed for 4 hours, and then further 12 hours formaturation at room temperature. The reaction mixture was extracted withtoluene, and the extract was washed with 1 N hydrochloric acid and thenwith 10% sodium hydroxide aqueous solution. Topping toluene gave 300 gof (E-2) represented by the following formula.

Example 3

To a solution of 216.3 g of diethyl n-butylmalonate dissolved in 700 mlof dry benzene was added a solution of 68.5 g of sodium ethoxidedissolved in 60 ml of dry ethanol in a 3-liter four-necked flask, andreaction was allowed for 15 minutes by heating under reflux. To thereaction liquid was added dropwise over 2 hours a solution of 239.3 g of(E-2) described above dissolved in 300 ml of dry benzene, and reactionwas allowed for 12 hours by heating under reflux. The reaction mixturewas extracted with ethyl ether, and treated with 10 N potassiumhydroxide and then with 6 N hydrochloric acid. Topping ethyl ether gavea tetracarboxylic acid. The tetracarboxylic acid thus obtained wasdissolved in pyridine, and decarboxylation by heating under reflux gavethe dicarboxylic acid (A0-3) represented by the following formula. 18.33g (0.05 mol) of the (A0-3) thus obtained was dissolved in 80 g ofethylene glycol, purged with ammonia gas. Termination of purging whenneutrality was reached gave a 100-g of a solution of the ammoniumdicarboxylate (B-3).

Example 4

286.5 g of diethyl 2-ethylhexylmalonate and 256.8 g 28% sodium methoxidesolution in methanol, and, separately, 224.1 g of diethylisopropylmalonate and 249.4 g of 28% sodium methoxide solution inmethanol, were stirred under heating in one and the other of two 1-literfour-necked flasks, respectively. Then, to 171.8 g of dichlorohexane ina 2-liter four-necked flask was added the whole volume of theabove-described mixtures of the malonic acid derivatives and sodiummethoxide methanol solutions, and reaction was allowed for 22 hours byheating under reflux. The reaction mixture was extracted with tolueneand treated with 10 N potassium hydroxide aqueous solution and then with6 N hydrochloric acid. Removal of the solvent gave a tetracarboxylicacid. Dissolution of the tetracarboxylic acid thus obtained in pyridineand decarboxylation by heating under reflux gave the dicarboxylic acid(A0-4) represented by the following formula. 18.31 g (0.05 mol) of(A0-4) thus obtained was dissolved in 80 g of ethylene glycol and purgedwith ammonia gas. Termination of purging when neutrality was reachedgave a 100-g solution of the ammonium dicarboxylate (B-4).

Example 5

In a 3-liter four-necked flask, the mixture of 383.9 g of diethyl1-hydroxyhexylmalonate and 331.2 g of 28% sodium methoxide solution inmethanol was heated with stirring. To the mixture liquid was added 114.5g of dichlorohexane, and reaction was allowed for 17 hours by heatingunder reflux. The reaction mixture was extracted with ethyl acetate, andtreated with 10 N potassium hydroxide aqueous solution and then with 6 Nhydrochloric acid. Removal of the solvent gave a tetracarboxylic acid.The tetracarboxylic acid thus obtained was dissolved in pyridine.Decarboxylation by heating under reflux gave the dicarboxylic acid(A0-5) represented by the following formula. 18.43 (0.05 mol) of (A0-5)thus obtained was dissolved in 80 g of ethylene glycol, and purged withammonia gas. Termination of purging when neutrality was reached gave a100-g solution of the ammonium dicarboxylate (B-5).

Preparation Example 3

The tosylated compound (E-3) [represented by the following formula(E-3)] from diethylene glycol mono methyl ether was synthesized in situ.In a 3-liter four-necked flask, a mixture of 610.6 g of diethyl malonateand 1500.2 g of 20% sodium ethoxide solution in ethanol was heated withstirring. To the mixture liquid was added dropwise the toluene solutionof the above-described (E-3). Reaction was allowed for 9 hours byheating under reflux, and the reaction mixture was extracted with ethylacetate. Removal of the solvent gave the diethyl malonate derivative(E-4) represented by the following formula.

Example 6

In a 2-liter four-necked flask, a mixture of 432.3 g of theabove-described (E-4) and 421.0 g of 20% sodium ethoxide solution inethanol was heated with stirring. To the mixture solution was addeddropwise 115.1 g of dichlorohexane, and reaction was allowed for 36hours by heating under reflux. The reaction mixture was extracted withethyl acetate, treated with 10 N potassium hydroxide aqueous solutionand then with 6 N hydrochloric acid. Removal of the solvent gave atetracarboxylic acid. The tetracarboxylic acid thus obtained wasdissolved in pyridine. Decarboxylation by heating under reflux gave thedicarboxylic acid (A0-6) represented by the following formula. 18.45 g(0.05 mol) of (A0-6) thus obtained was dissolved in 80 g of ethyleneglycol, and purged with ammonia gas. Termination of purging whenneutrality was reached gave a 100-g solution of the dicarboxylic acid(B-6).

Preparation Example 4

The tosylated compound (E-5) [represented by the following formula(E-5)] from diethylene glycol monophenyl ether was synthesized in situ.In a 3-liter four-necked flask, a mixture of 610.6 g of diethyl malonateand 1500.2 g of 20% sodium ethoxide solution in ethanol was heated withstirring. To the mixture liquid was added dropwise a toluene solution ofthe above-described (E-5). Reaction was allowed for 9 hours by heatingunder reflux, and the reaction mixture was extracted with ethyl acetate.Removal of the solvent gave the diethyl malonate derivative (E-6)represented by the following formula.

Example 7

In a 2-liter four-necked flask, the mixture of 534.6 g of theabove-described (E-6) and 421.0 g of 20% sodium ethoxide solution inethanol was heated with stirring. To the mixture liquid was addeddropwise 115.1 g of dichlorohexane. Reaction was allowed for 36 hours byheating under reflux. The reaction mixture was extracted with ethylacetate, and treated with 10 N potassium hydroxide aqueous solution andthen with 6 N hydrochloric acid. Removal of the solvent gave atetracarboxylic acid. The tetracarboxylic acid thus obtained wasdissolved in pyridine. Decarboxylation by heating under reflux gavedicarboxylic acid (A0-7) of Example 7 represented by the followingformula. 18.79 g (0.04 mol) of the (A0-7) thus obtained was dissolved in80 g of ethylene glycol, and purged with ammonia gas. Termination ofpurging when neutrality was reached gave a 100-g solution of theammonium dicarboxylate (B-7).

Comparative Example 1

18.04 (0.06 mol) of the n-octadecane dicarboxylic acid represented bythe following formula [manufactured by Tokyo Kasei Kogyo Co., Ltd.](A0-8′) was dissolved in 80 g of ethylene glycol, and purged withammonia gas. Termination of purging when neutrality was reached gave a100-g solution of the ammonium dicarboxylate (B-8′).

Comparative Example 2

To a solution of 216.3 g of diethyl methylmalonate dissolved in 700 mlof dry benzene was added a solution of 68.5 g of sodium ethoxidedissolved in 60 ml of dry ethanol. Reaction was allowed for 15 minutesby heating under reflux. To the reaction liquid was added dropwise over2 hours a solution of 116.2 g of 1,6-dibromohexane dissolved in 190 mlof dry benzene. Reaction was allowed for 12 hours by heating underreflux. Extraction of the reaction mixture with ethyl ether, which wasfollowed by treatment with 10 N potassium hydroxide and then with 6 Nhydrochloric acid, gave a tetracarboxylic acid. Dissolution of the thusobtained tetracarboxylic acid in pyridine, followed by decarboxylationby heating under reflux, gave 2,9-dimethylsebacic acid (A0-9′)represented by the following formula. 17.42 (0.08 mol) of2,9-dimethylsebacic acid (A0-9′) thus obtained was dissolved in 80 g ofethylene glycol, and purged with ammonia gas. Termination of purgingwhen neutrality was reached gave a 100-g solution of the ammoniumdicarboxylate (B-9′).

Calculation of Physicochemical Parameters of Carboxylate Anions

Van der Waals volume was calculated for the carboxylate anions (A) ofthe above-described Examples and Comparable Examples using the MM3/PM3method of the CAChe system. The results are shown in Table 1.

The solubility parameter of the carboxylate anions (A) and that of theorganic polar solvents (C), which are calculated by the Fedors method,are shown in Table 1 along with the modulus of the difference betweenthem.

The maximal value of electron density of the highest occupied molecularorbital for the carboxylate anions (A) calculated by the MM3/PM3 methodof the CAChe system is shown in Table 1.

The value of energy of formation for the ionic complexes (D) from thecarboxylate anions (A) and aluminum ion calculated by the MM3/PM3 methodof the CAChe system is shown in Table 1.

Measurement of Specific Electric Conductivity and Spark Voltage forElectrolytic Solutions

Measurement was made for the specific electric conductivity and thespark voltage for the electrolytic solutions of Examples 1-7[(B-1)-(B-7), respectively] and for the electrolytic solutions ofComparative Examples 1-2 [(B-8′)-(B-9′), respectively] by the followingmethods. The results are shown in Table 1.

Specific Electric Conductivity: The specific electric conductivity wasmeasured at 30° C. using an electric conductivity meter CM-40Smanufactured by To a Denpa K.K.

Spark Voltage: Discharge voltage was measured for the electrolyticsolutions using an etched and anodically oxidized aluminum foil forhigh-voltages of a size of 10 cm² and applying a constant current (2mA). TABLE 1 Van der Modulus of Maximal value Vaals SolubilitySolubility difference of density of Formation Specific volume parameterparameter between the highest energy electric Spark of (A) of (A) of (C)(A) and (C) occupied of (D) conductivity voltage (Angstrom³)(cal^(1/2)cm^(−3/2)) (cal^(1/2)cm^(−3/2)) (cal^(1/2)cm^(−3/2)) orbitalof (A) (kcal/mol) (mS/cm) (V) Example 1 230.6 12.8 17.8 5.0 0.16 −393.31.4 508 Example 2 193.5 13.5 17.8 4.3 0.17 −434.1 1.4 509 Example 3278.2 10.0 17.8 7.8 0.32 −273.8 1.3 504 Example 4 264.9 9.6 17.8 8.20.08 −262.2 0.9 538 Example 5 286.8 11.7 17.8 6.1 0.13 −302.2 1.1 516Example 6 274.6 10.0 17.8 7.8 0.16 −264.2 1.0 510 Example 7 262.8 10.617.8 7.2 0.16 −305.5 1.0 578 Comparative 228.9 10.1 17.8 7.7 0.11 −216.00.9 476 example 1 Comparative 154.5 10.5 17.8 7.3 0.04 −184.0 1.3 436example 2

As evident from Table 1, the electrolytic solutions of Examples 1-7 ofthe present invention generate spark voltages that are higher than thosegenerated with the electrolytic solutions of Comparative Examples 1-2.

INDUSTRIAL APPLICABILITY

As the electrolytic solution of the present invention for electrolyticcapacitors allows to generate higher spark voltages without inducingdecrease in the specific electric conductivity, it is of greatindustrial applicability, for its use in electrolytic capacitors,especially of moderate/high voltage classes, will realize space-savingaround installed capacitors together with higher reliability ofcapacitors.

1. An electrolytic solution comprising an organic polar solvent (C) andfurther comprising a carboxylic acid (A0) and/or a carboxylate salt (B),wherein the carboxylate anion (A) is one with which the energy offormation in water of the ionic complex (D) with aluminum ion is notmore than −250 kcal/mol and not less than −500 kcal/mol as calculated bythe MM3/PM3 method of the CAChe system.
 2. The electrolytic solution ofclaim 1 wherein the van der Waals volume of the carboxylate anion (A) isnot less than 190 cubic angstroms and not more than 500 cubic angstroms.3. The electrolytic solution of claim 1 or 2 wherein the modulus of thedifference between the solubility parameter as calculated by Fedorsmethod for the carboxylate anion (A) and the solubility parameter forthe organic polar solvent (C) is not less than 4 and not more than
 9. 4.The electrolytic solution of one of claims 1 to 3 wherein the maximalelectron density of the highest occupied orbital of the molecule of thecarboxylate anion (A), as calculated by the MM3/PM3 method of the CAChesystem, is not less than zero and not more than 0.5.
 5. The electrolyticsolution of one of claims 1 to 4 wherein the carboxylate anion (A) is asecondary dicarboxylate dianion.
 6. The electrolytic solution of one ofclaims 1 to 5 wherein the carboxylate anion (A) is represented by thefollowing general formula (1)

wherein X is a linear or branched, saturated or unsaturated, divalenthydrocarbon group having 1 to 12 carbon atoms, and may have an etherbond, R₁ and R₂ are linear or branched, saturated or unsaturated,monovalent hydrocarbon groups having 2 to 10 carbon atoms wherein R₁ andR₂ are different from each other, or R₁ and R₂ are monovalenthydrocarbon groups having an ether bond and having 2 to 10 carbon atomswherein R₁ and R₂ may be the same or different from each other, or bythe following general formula (2)

wherein Y is a linear or branched, saturated or unsaturated, divalenthydrocarbon group having 1 to 12 carbon atoms, and may have an etherbond, R₃ and R₄ are linear of branched, saturated or unsaturated,divalent hydrocarbon groups having 2 to 10 carbon atoms, and may have anether bond, wherein R₃ and R₄ may be the same or different from eachother.
 7. The electrolytic solution of one of claims 1 to 6 wherein theionic complex (D) formed from the carboxylate anion (A) and aluminum ionis represented by the following general formula (3),

wherein X is a linear or branched, saturated or unsaturated, divalenthydrocarbon group having 1 to 12 carbon atoms, and may have an etherbond, R₁ and R₂ are linear or branched, saturated or unsaturated,monovalent hydrocarbon groups having 2 to 10 carbon atoms wherein R₁ andR₂ are different from each other, or R₁ and R₂ are monovalenthydrocarbon groups having an ether bond and having 2 to 10 carbon atomswherein R₁ and R₂ may be the same or different from each other, or bythe general formula (4),

wherein Y is a linear or branched, saturated or unsaturated, divalenthydrocarbon group having 1 to 12 carbon atoms, and may have an etherbond, R₃ and R₄ are linear of branched, saturated or unsaturated,divalent hydrocarbon groups having 2 to 10 carbon atoms, and may have anether bond, wherein R₃ and R₄ may be the same or different from eachother.
 8. The electrolytic solution of one of claims 1 to 7 wherein thecarboxylate salt (B) is an ammonium salt and/or an amine salt.
 9. Theelectrolytic solution of one of claims 1 to 8 wherein the organic polarsolvent (C) is ethylene glycol.
 10. The electrolytic solution of one ofclaims 1 to 9 to be used in an electrolytic capacitor.